1. Field of the Invention
This invention relates to polystyrene resin compositions which possess excellent self-extinguishing properties, as well as thermal resistance. The term "self-extinguishing" means that the resin composition burns for a limited time after removal of a test flame, but the resin does not support combustion and it self-extinguishes soon after the flame is removed.
2. Description of the Prior Art
Polystyrene resin compositions, because of their many excellent characteristics, are used in various fields in large quantities. On the other hand, because of their ease of combustion, these polystyrene compositions cannot be said to be the best material for use when non-flammability is an essential requirement. In fact, polystyrene resin compositions are unsuitable for making parts of electric appliances or business machines, because the parts may burn and cause serious trouble.
Further, the prior polystyrene resin compositions, when molded into foamed products, will have the foregoing disadvantages of being readily combustible so magnified as to prohibit their use as construction materials, even though they are very excellent in their thermal insulation and sound-absorbing properties.
A variety of methods have been proposed for eliminating the foregoing disadvantages and to confer a fire-retardant characteristic on polystyrene resin compositions. For instance, it has been suggested that the addition of one or more of such additives as a halogen-containing compound, a phorphorus-containing compound and antimony trioxide, is effective in imparting fire-retardancy to polystyrene resin compositions, and that these substances can be blended with polystyrene resins or mixed in advance with styrene monomer or styrene mixed monomers and then polymerized. Further, it has also been published that a halogen-, or phosphorus-containing monomer is synthesized and then copolymerized with styrene monomer to form directly fire-retardant polystyrene resin compositions.
The troubles involved in these prior methods, however, are that the resulting resin compositions have decreased impact-resistance and they also soften at lower temperatures, so that they must be extruded or molded only under a controlled narrow range of temperature. Consequently, if the temperature control should go wrong, the above additive compounds are apt to partially decompose to produce colored pellets and molded products, or to degrade to become lower in their fire-retarding effect. Also, if these additive compounds which are very expensive are added in a sufficiently large amount as to confer a good fire-retarding effect on polystyrene resin compositions, then the finished products themselves are high in cost and inferior in their mechanical properties.
Under these circumstances, in order to produce new resin compositions completely free of the foregoing defects, efforts have so far been directed toward discovering methods and agents capable of conferring a good flame-retarding property on polystyrene resin compositions.
Fire-retardant agents merely containing higher contents of halogen or phosphorus, however, do not always confer greater fire retardance on polystyrene resin compositions. Moreover, the resulting products which simply contain a larger quantity of halogen or phosphorus are not necessarily higher in fire retardance. The agents further vary in their fire-retarding effect when they differ in molecular structure, even though the atoms constituting the agents are the same in number and type. Therefore, it is very dificult to estimate in advance the possible fire-retarding effect of specific agents. In other words, various combinations of different fire-retarding agents or of the agents with other additives might produce a fire-retarding effect, but the determination of ideal combinations of agents is extremely difficult.
We previously found that a mixture of (a) tricresyl phosphate (TCP) and/or triphenyl phosphate (TPP) with (b) tetrabromobisphenol A (TBA), added in an equimolar mixture of (a) and (b), provides surprisingly excellent fire-retardance, as described in U.S. patent application Ser. No. 394,690, now U.S. Pat. No. 3,879,345 filed Sept. 6, 1973, the entire contents of which are incorporated herein by reference.
In our prior invention, if TBA oligomer is used, instead of TBA monomer, in equimolar mixture with (a) TCP and/or TPP, and regarding each of the TBA moieties which constitute the oligomer as one unit for the purpose of the molar ratio calculation, a great fire-retarding effect is conferred on the resulting resin composition and the composition is markedly improved in thermal resistance as well.